Workspace grey blank
Look at the top. It says "Back to Previous" -> That's how you get back.

What happened is that you accidentally hit Shift + Space Bar. This makes the current View go full screen. (The one your mouse is over)
Alternatively you can just hit Shift + Space Bar again.

EDIT:

On the other hand, if you have trouble finding the geometry in your 3D view, you can either use Shift + C as pointed out by @HenrikD or, press "." or "," on your Num Pad to center the view on a selected object.